The present invention relates generally to the art of producing printed circuits and the like by photolithographic processes using photomasks, and relates more particularly to the art of facilitating repeated use of a photomask assembly comprising a pair of glass plates.
A printed circuit board comprises a substrate of insulator material and a circuit pattern of conductive material commonly formed by photolithographic processes. For example, a fiberglass board to be printed with circuitry is plated with copper and coated with a layer of photoresist material, a photosensitive polymer. The coated surface is then exposed to actinic radiation through a photomask master, a plate with some areas which are transparent to the actinic radiation and some areas which are not, which defines the desired circuit pattern. The pattern is developed as a relief image in the photoresist by solvent treatment, the exposed and unexposed portions of the photosensitive polymer having different solubilities. The printed substrate may be either a positive or negative image of the photomask master depending on whether a positive or negative working photopolymer is used. Typically, the unexposed photoresist is removed and the underlying copper etched away to leave a pattern of conductive copper in the image of the transparent pattern of the photomask.
Photomasks used in photolithographic processes such as the manufacture of printed circuits are preferably glass plates coated with photographic emulsion, iron oxide or chromium having a pattern of transparent and opaque areas (transparent and opaque with respect to actinic radiation). These photomasks must be repeatedly registered in relation to photoresist coated substrates to accurately reproduce the desired pattern. For this reason, durable but opaque films such as chromium may be less preferred than less durable but transparent films (transparent and opaque with respect to visible light). The visually aligned assembly is exposed to radiation which passes through the transparent areas of the photomask to photographically impose a pattern on the coated substrate, which is typically developed as a relief image by dissolution of the unexposed photosensitive material. However, in the alternative, the exposed coating material may be removed by dissolution, or either the exposed or unexposed areas may be chemically treated. The chemistry of photolithographic processes is well-known, and any of the conventional routes is acceptable to arrive at a circuit pattern on the substrate.
For high volume production, more durable photomasks have been developed wherein the pattern is a stained image within a glass plate, rather than a coating. These photomasks may be used many times in a photolithographic imaging process to produce high resolution patterns. In many current applications, it is also useful to reduce the size of printed circuit boards by increasing the density of conductive lines, which requires increasing resolution, or by producing patterns on both sides of a substrate. Both increasing the resolution of a dense pattern and simultaneously printing both sides of a circuit board may be accomplished using a pair of glass photomasks precisely registered on opposite sides of a photoresist coated substrate.
When both sides of photoresist coated substrates are to be printed simultaneously in a high volume commercial operation, a durable dual photomask assembly which can be repetitiously aligned quickly and accurately with minimum handling and risk of breakage of the glass plate is required.
An apparatus typically employed in the photolithographic processes described above generally comprises a pair of frames defining an enclosed space which can be evacuated. A pair of flexible photolithographic masters may be visually aligned and manually secured individually to the frame members by such devices as tape, clamps or spring clips. However, glass photomasks are typically registered using cylindrical alignment pins fitted through holes drilled through corresponding areas of the glass plates. A photopolymer coated substrate is precisely registered between the photomasks, and the assembly is subjected to vacuum and exposed to actinic radiation. When the substrate is removed from the assembly to develop the circuit pattern, the top glass plate must be lifted vertically and set aside until it is replaced over a fresh substrate. This process is costly, time-consuming and inaccurate, with the glass exposed to high risk of breakage. Therefore, improvements to the visual-manual registration technique have been sought.
For example, U.S. Pat. No. 4,032,233 to Oscarsson et al describes an apparatus which facilitates the registry of image bearing transparencies with a plate having a photosensitive coating thereon. The registry apparatus includes upper and lower frames forming an enclosure on which the plates and transparencies are placed in sandwich relation. An outer gasket between the frame permits a vacuum to be applied to the enclosed space, while an inner perimeter channel permits a separate vacuum to hold the transparencies in place while the frames are opened to remove, insert or replace a plate.
U.S. Pat. No. 3,927,943 to Pohl et al discloses contact printing on opposite sides of a sensitized substrate by mounting first and second masks in first and second support members, adjusting the first support member to align the masks with respect to a common alignment pin, removing the second support member, mounting the substrate in a third support member, aligning the substrate with the first mask, forcing the substrate into contact with the first mask and magnetically holding it in place, removing the third support member, fastening the second support member to the first support member with the alignment pin, and drawing a vacuum between the masks to bring them into contact with opposite sides of the substrate.